JPH01129842A - Nuclear magnetic resonance imaging apparatus - Google Patents

Abstract

PURPOSE:To sufficiently cut off the circuit of an irradiation coil system, by constituting a parallel resonance circuit of the resonating inductance, which is parallelly connected to the cross diode inserted in the irradiation coil of the irradiation coil system in series, and a variable condenser. CONSTITUTION:In an irradiation coil system 20a, a resonance frequency adjusting capacity 2 and an impedance adjusting capacity 3 are connected to an irradiation coil 1 and a cross diode 5 is inserted in the irradiation coil 1. A resonating inductor 29 and a resonance frequency adjusting variable condenser 30 are connected to the cross diode 5 in parallel to constitute a parallel resonance circuit. When the NMR signal discharged from an examinee 9 is received, the parallel resonance circuit is resonated in parallel to the NMR signal at the time of reception inclusive of the connection capacity C of the cross diode 5 and the connection capacity C is negated to become only the parallel resistance component of the cross diode 5. As a result, the circuit of the irradiation coil system 20a is sufficiently cut off and the connection thereof to a receiving coil system 20b is sufficiently prevented.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to nuclear magnetic resonance imaging, which obtains a tomographic image of a desired part of a subject (human body) by utilizing the nuclear magnetic resonance (hereinafter abbreviated as rNMRJ) phenomenon. The present invention relates to an apparatus, and particularly to a nuclear magnetic resonance imaging apparatus that can sufficiently prevent coupling between a receiving coil system and an irradiating coil system when detecting high frequency signals emitted from a subject by nuclear magnetic resonance.

[Conventional technology]

A nuclear magnetic resonance imaging apparatus includes a magnetic field generating means that applies a static magnetic field and a gradient magnetic field to a subject, and an irradiation coil that irradiates high-frequency signals to cause nuclear magnetic resonance in the nuclei of atoms constituting the living tissue of the subject. a receiving system having a receiving coil system that detects the high-frequency signals emitted by the above-mentioned nuclear magnetic resonance; and a signal processing system that performs image reconstruction calculations using the high-frequency signals detected by the receiving system. It is equipped with. Here, in a nuclear magnetic resonance imaging apparatus that examines a human body, the subject is irradiated with spatially wide-variant high-frequency signals, and the high-frequency signal emitted from the subject by nuclear magnetic resonance ( In order to detect this (called NMR signal) with high sensitivity < good S/H, a relatively large irradiation coil is used in the irradiation coil system of the transmission system, and a relatively large irradiation coil is used in the reception coil system of the reception system. uses a relatively small receiving coil placed close to the subject. If the irradiation coil is placed in the same direction as the receiving coil, the two will be coupled at high frequencies and the detection sensitivity will be lowered.Therefore, the two are usually placed on axes perpendicular to each other.

However, as the receiving coil, an orthogonal coil in which two coils are orthogonally combined or a surface coil in which one coil is set close to the body surface in an arbitrary direction may be used. At this time, in the orthogonal coils, one of the two coils is always arranged in the same direction as the irradiation coil. Furthermore, since the surface coil is set in any direction depending on the region to be examined, it may not be possible to arrange it on the axis orthogonal to the irradiation coil. Therefore, it is necessary to take measures to forcibly prevent the coupling between the receiving coil and the irradiating coil.

Therefore, conventionally, as shown in FIG. 3, in an irradiation coil system 4 in which a resonant frequency adjustment capacitor 2 and an impedance adjustment capacitor 3 are connected to an irradiation coil 1, a cross diode 5 is inserted in series with the irradiation coil 1. However, the circuit of the irradiation coil system 4 is cut off to prevent the irradiation coil 1 from coupling to the reception coil 6 except when irradiating a high-frequency signal.

[Problem that the invention seeks to solve]

However, in the irradiation coil system 4 of such a conventional device, since the cross diode 5 always has a junction capacitance C, the irradiation coil system 4 becomes conductive with the junction capacitance C, and the irradiation coil It was not possible to completely shut off system 40 circuits. Therefore, the coupling of the irradiating coil 1 to the receiving coil 6 could not be sufficiently prevented.

Further, as the cross diode 5, one must be used that can withstand the large current of the irradiation output, and as a result, the junction capacitance C of the loss diode 5 becomes a relatively large value. Therefore, it becomes increasingly difficult to prevent the irradiation coil 1 from being coupled to the reception coil 6. For these reasons, simply inserting the cross diode 5 in series with the irradiation coil 1 cannot completely shut off the irradiation coil system 4, and cannot sufficiently prevent the coupling between the irradiation coil 1 and the reception coil 6. When both are arranged other than orthogonally, the Q value decreases and the detection sensitivity of the receiving coil 6 decreases significantly. moreover.

When receiving an NMR signal from a subject, the irradiation coil system 4 is induced by external noise, which lowers the S/N of the reception coil system and deteriorates the quality of the obtained tomographic image.

Therefore, an object of the present invention is to provide a nuclear magnetic resonance imaging apparatus that can solve such problems.

[Means for solving problems]

Means of the present invention for solving the above problems includes a magnetic field generating means for applying a static magnetic field and a gradient magnetic field to a subject.

A transmission system includes an irradiation coil system that irradiates radio frequency (No. 73) to cause nuclear magnetic resonance in the nuclei of atoms constituting the living tissue of the subject, and detects the radio frequency signal emitted by the nuclear magnetic resonance. In a nuclear magnetic resonance imaging apparatus, the nuclear magnetic resonance imaging apparatus includes a receiving system having a receiving coil system, and a signal processing system that performs image reconstruction calculations using high-frequency signals detected by the receiving system. A parallel resonant circuit is constructed by connecting a resonant inductor and a variable capacitor in parallel with a cross diode inserted in series with the irradiation coil that irradiates the signal, and the high-frequency signal emitted from the subject is detected by the receiving coil system. In some cases, this is done using a nuclear magnetic resonance imaging device in which the irradiation coil system is shut off.

[For production]

The nuclear magnetic resonance imaging device configured in this way is
A parallel resonant circuit is configured by a cross diode inserted in series with the irradiation coil of the irradiation coil system, and a resonant inductance and a variable capacitor connected in parallel. When receiving an NMR signal from the subject, the parallel resonant circuit resonates, so that the portion including the junction capacitance of the cross diode becomes high impedance, and the circuit of the irradiation coil system is sufficiently cut off.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a block diagram showing the overall configuration of a nuclear magnetic resonance imaging apparatus according to the present invention. This nuclear magnetic resonance imaging apparatus obtains a tomographic image of a subject by using the nuclear magnetic resonance (NMR) phenomenon, and as shown in FIG. 11, a sequencer 12, a transmission system 13, a magnetic field gradient generation system 14, a reception system 15, and a signal processing system 16.

The static magnetic field generating magnet 10 generates a strong and uniform static magnetic field around the subject 9 in the body axis direction or in a direction perpendicular to the body axis, and is used to generate a strong and uniform static magnetic field around the subject 9 in a certain expanse of space around the subject 9. A magnetic field generating means of a permanent magnet type, a constant electromotive force type, or a super electromotive force type is arranged in the magnetic field generating means. The sequencer 12 operates under the control of CFULL and sends various commands necessary for data collection of tomographic images of the subject 9 to the transmission system 13, the magnetic field gradient generation system 14, and the reception system 15.

The transmission system 13 irradiates high-frequency signals to cause nuclear magnetic resonance in the nuclei of atoms constituting the living tissue of the subject 9, and includes a high-frequency oscillator 17, a modulator 18, a high-frequency amplifier 19, and an irradiation coil system. 20a, the high frequency pulse outputted from the high frequency oscillator 17 is amplitude modulated by the modulator 18 according to the command of the sequencer 12, and after this amplitude modulated high frequency pulse is amplified by the high frequency amplifier 19, it is brought close to the subject 9. Irradiation coil system 20 arranged as
By supplying the high frequency signal to a, the subject 9 is irradiated with the high frequency signal.

The above magnetic field gradient generation system 14. consists of gradient magnetic field coils 21 wound in the three axial directions of x, y, and z, and a gradient magnetic field power supply 22 that drives each coil.

By driving the gradient magnetic field power supply 22 of each coil according to the command from the sequencer 12, x, y,
A gradient magnetic field G x + G V tGz in the triaxial direction of z is applied to the subject 9 . A slice plane for the subject 9 can be set by applying this gradient magnetic field. The receiving system 15 receives a high frequency signal (N
The device detects the response of the subject 9 due to the high frequency signal irradiated from the irradiation coil system 20a, and is composed of a receiving coil system 20b, an amplifier 23, a quadrature phase detector 24, and an A/D converter 25. The NMR signal is detected by a receiving coil system 20b placed close to the subject 9, and sent to the A/D converter 2 via an amplifier 23 and a quadrature phase detector 24.
5, it is converted into a digital quantity, and then it is input to the quadrature phase detector 2 at the timing according to the command from the sequencer 12.
4, and the signal is sent to a signal processing system 16. This signal processing system 16 consists of a CPU 1, a storage device such as a magnetic disk 26 and a magnetic tape 27, and a display 28 such as a CRT. The signal intensity distribution of an arbitrary cross section or the distribution obtained by performing appropriate calculations on a plurality of signals is converted into an image and displayed as a tomographic image on the display 28. In addition, in FIG. 1, the irradiation coil system 20a and the reception coil system 20b
Further, the gradient magnetic field coil 21 is arranged within the magnetic field space of the static magnetic field generating magnet 10 arranged in the space around the subject 9.

Here, in the irradiation coil system 20a, as shown in FIG. 2, a resonant frequency adjustment capacitor 2 and an impedance adjustment capacitor 3 are connected to the irradiation coil 1, and a cross diode 5 is connected in series to the irradiation coil 1. It has been inserted. Further, in the present invention, a resonant inductor 29 and a variable capacitor 30 for adjusting the resonant frequency are connected in parallel with the cross diode 5 to form a parallel resonant circuit. When receiving the NMR signal emitted from the subject 9, the parallel resonant circuit is connected to the cross diode 5.
By causing parallel resonance to the NMR signal during reception, including the junction capacitance C, the junction capacitance C is canceled and only the parallel resistance component of the cross diode 5 remains, increasing its impedance value to about 10 times, for example. Can be done.

As a result, the circuit of the irradiation coil system 20a is sufficiently cut off, and the coupling between the irradiation coil system 20a and the reception coil system 20b is also sufficiently prevented.

〔Effect of the invention〕

Since the present invention is configured as described above, when receiving the NMR signal emitted from the subject 9, the parallel resonant circuit composed of the cross diode 5, the resonant inductor 29, and the variable capacitor 30 is connected to the irradiation coil system 20a. By causing resonance, the interjunction capacitance C of the cross diode 5 can be canceled out and its impedance can be increased, and the circuit of the irradiation coil system 20a can be sufficiently interrupted. Therefore, the receiving coil system 20b and the irradiating coil system 2
Bonding with Oa can be sufficiently prevented. From this, even if the irradiation coil 1 of the irradiation coil system 20a and the reception coil 6 of the reception coil system 20b are arranged in any direction other than the orthogonal direction, the Q value does not decrease and the reception coil 6
It is possible to prevent the detection sensitivity from decreasing. Furthermore, since the circuit of the irradiation coil system 20a is cut off when receiving the NMR signal from the subject 9, induction of external noise by the irradiation coil system 20a can be avoided.
The S/N of the receiving coil system 20b can be improved.

For these reasons, the quality of the obtained tomographic image is improved and good diagnostic information can be obtained.

4.13! ! Brief explanation of page 1 Fig. 1 is a block diagram showing the overall configuration of the nuclear magnetic resonance imaging apparatus according to the present invention, Fig. 2 is a circuit diagram showing the internal structure of the irradiation coil system in the present invention, and Fig. 3 is a conventional apparatus. FIG. 2 is a circuit diagram showing the internal configuration of an irradiation coil system in FIG.

1... Irradiation coil, 5... Cross diode,
6... Receiving coil, 9... Subject, 10...
Static magnetic field generating magnet, 11...Central processing unit (CPU
).

12...Sequencer, 13...Transmission system, 1
4... Magnetic field gradient generation system, 15... Receiving system, 1
6... Signal processing system, 20a... Irradiation coil system,
20b... Receiving coil system, 29... Resonant inductor, 30... Variable capacitor, C... Junction capacitance of cross diode.

Claims (1)

[Claims] a magnetic field generating means for applying a static magnetic field and a gradient magnetic field to the subject;
a transmission system having an irradiation coil system that irradiates high-frequency signals to cause nuclear magnetic resonance to the nuclei of atoms constituting the living tissue of the subject; and a reception system that detects the high-frequency signals emitted by the nuclear magnetic resonance. In a nuclear magnetic resonance imaging apparatus comprising a receiving system having a coil system and a signal processing system that performs image reconstruction calculations using the high frequency signals detected by the receiving system, the high frequency signals are received by the irradiation coil system. A parallel resonant circuit is constructed by connecting a resonant inductor and a variable capacitor in parallel with a cross diode inserted in series with the irradiation coil, and is used to detect high-frequency signals emitted from the subject in the receiving coil system. A nuclear magnetic resonance imaging apparatus characterized in that the irradiation coil system is shut off.